1. Field of the Invention
The present invention relates to a method and apparatus for detecting an impact to actuate a vehicle passenger restraint system, and more particularly to a restraint system such as an air bag system, a seat belt pre-loader or the like.
2. Description of the Prior Art
In recent years, air bag systems have been installed in vehicles as a restraint system to supplement a seat belt. That is, the air bag system is provided for reducing the shock to a driver in the case of a frontal impact on the vehicle exceeding a predetermined level. According to a conventional air bag system, when an acceleration sensor detects a frontal impact greater than a predetermined threshold level, an air bag placed inside a steering wheel is momentarily inflated to reduce the shock to the driver. In such a restraint system, it is important to detect an impact on the vehicle accurately and rapidly. Therefore, an improvement of the acceleration sensor is required and an improvement in the method and apparatus for detecting an impact force according to the nature of the impact is also required.
As to the judging of the nature of an impact on the vehicle, a method for judging whether such an impact was made by a collision or a bad road condition is disclosed in Japanese Patent Laid-open Publication No. Hei 4 (1992)-358945. In this method, a first value is obtained by integrating an acceleration signal from an acceleration sensor for a certain period of a section, a second value is obtained by differentiating the acceleration signal and a third value is obtained by adding the first value to the second value. An impact judgment for a restraint system is made based on the third value.
However, the above-mentioned method uses a sectional integration, whereby the method needs a certain amount of time (e.g., 30-60 ms) for judging an impact accurately. Since this method takes a long time to judge whether the impact was made due to a bad road condition, it is difficult to immediately judge an impact in a vehicle where the demand time of an impact judgment is short. If a sectional integrated time was made shorter, the threshold level of judging a bad road condition would be increased and the impact judgment would be too late. In this method, it is difficult to achieve an acceptable basic performance.